Use full_res instead of expect_full_res

compiler/rustc_resolve/src/late.rs

@@ -1969,7 +1969,7 @@ impl<'a: 'ast, 'b, 'ast> LateResolutionVisitor<'a, 'b, 'ast> {
                     None
                 }
             })
-            .map(|res| res.expect_full_res())
+            .and_then(|res| res.full_res())
             .filter(|res| {
                 // Permit the types that unambiguously always
                 // result in the same type constructor being used

src/test/ui/resolve/issue-102946.rs

@@ -0,0 +1,7 @@
+impl Error for str::Utf8Error {
+    //~^ ERROR cannot find trait `Error` in this scope
+    //~| ERROR ambiguous associated type
+    fn description(&self)  {}
+}
+
+fn main() {}

src/test/ui/resolve/issue-102946.stderr

@@ -0,0 +1,26 @@
+error[E0405]: cannot find trait `Error` in this scope
+  --> $DIR/issue-102946.rs:1:6
+   |
+LL | impl Error for str::Utf8Error {
+   |      ^^^^^ not found in this scope
+   |
+help: consider importing this trait
+   |
+LL | use std::error::Error;
+   |
+
+error[E0223]: ambiguous associated type
+  --> $DIR/issue-102946.rs:1:16
+   |
+LL | impl Error for str::Utf8Error {
+   |                ^^^^^^^^^^^^^^
+   |
+help: you are looking for the module in `std`, not the primitive type
+   |
+LL | impl Error for std::str::Utf8Error {
+   |                +++++
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0223, E0405.
+For more information about an error, try `rustc --explain E0223`.